A Patent Literature 1 listed below discloses a mounting structure in which a sonar sensor for detecting a distance to a detection object is mounted in a bumper without degrading its appearance. In the mounting structure, the sonar sensor is attached, by a holder, to a through hole opened on the bumper so that a detection plane of the sonar sensor and an outer surface of the bumper form a continuous smooth surface (flush surface). Since the detection plane of the sonar sensor and the outer surface of the bumper form a continuous smooth surface (flush surface), its appearance looks good. In addition, in the mounting structure, the detection plane and the bumper are painted in the same color in order to make its appearance better.